Vertical field type MRI apparatus with a conical cavity situated in the main magnet

ABSTRACT

The invention relates to a vertical field type MRI apparatus provided with a superconducting coil system  20   a,    20   b  for generating a substantially homogeneous magnetic field in an imaging volume  18  of the apparatus. The coil system includes a circular outer coil  28  and a supplementary coil  30  which is positioned in the same plane  32  as and within the outer coil, which coils conduct opposite currents. The ratio D a /D o  of the diameter D a  of the supplementary coil to the diameter D o  of the outer coil lies between 0,7 and 0,9. Further coils  34  to  38, 40  to  46  for making the field even more homogeneous are preferably located on a conical surface  48, 50  within the first mentioned coils  28, 30  in such a manner that a recess is formed in which a conically shaped gradient coil system  52  can be accommodated, with the result that the expensive outer coil  28  and the supplementary coil  30  can be arranged at an as short as possible distance from the space for receiving the patient to be examined.

BACKGROUND OF INVENTION

The invention relates to a vertical field type MRI apparatus for formingmagnetic resonance images, including:

at least one field generating superconducting coil system for producinga substantially homogeneous magnetic field in an imaging volume of theapparatus,

which coil system includes:

a round outer coil that is situated in an outer coil plane;

a round supplementary coil that is situated within the outer coil.

An apparatus of this kind is known from U.S. Pat. No. 5,939,962. Thehomogeneous magnetic field required for MR imaging in such a verticalfield type apparatus is usually generated by two oppositely situatedmagnetic poles wherebetween the patient to be examined can be arranged.Generally speaking, said magnetic field then has a vertical direction.Apparatus of this kind offers the advantage that the patient keeps acomparatively broad view of the surroundings when arranged in such anapparatus, so that sensations of claustrophobia occur less frequently.

An iron circuit that is capable of transporting the complete fluxthrough the system becomes very heavy in the case of magnet systemshaving a field strength beyond approximately 0.5 T. A sensiblealternative in that case is to omit the iron circuit completely and toconstruct the magnet system as an actively shielded air coil system. Inthat case there are no poles in the sense of iron structures that boundthe space of the magnet system that is accessible to the patient, butthe surfaces of the magnet system that bound the patient space will alsobe referred to hereinafter as “poles” for the sake of simplicity. Forfield strengths beyond 0.5 T the coils must be constructed so as to besuperconducting. They are kept at the operating temperature in acryostat. The “poles” are then formed by the outer wall of the vacuumenvelope of the cryostat.

The cited United States patent discloses a superconducting coil systemwhich consists of a round outer coil (a so-called “side coil” that isdenoted by the reference 12 a therein), a round supplementary coil(referred to therein as the “fourth coil” which bears the reference 12d), and a number of further coils (referred to as the “second and thethird coils” bearing the references 12 b and 12 c therein). Thehomogeneous field in the imaging volume is generated mainly by the firsttwo coils 12 a and 12 b and the other coils mentioned superpose afurther homogenizing field thereon.

As is generally known, and also described in the cited U.S. patent, forthis type of apparatus the aim is to arrange the field generating coilin the upper magnetic pole at an as small as possible distance from thefield generating coil in the lower magnetic pole. This aim stems fromthe fact that the production costs of such a system increase byapproximately a power of five of said pole distance, so that it isadvantageous to keep this distance as small as possible. Because of thisaim, the outer coils in the known apparatus are arranged practicallydirectly against the boundary of the freely accessible space between themagnetic poles.

When the outer coils are mounted in that manner, the gradient coils inthe known apparatus must extend to practically the diameter of theassociated outer coils because of the necessary linearity of thegradient field in the imaging volume. Consequently, room for thesupplementary coil (also having a voluminous and heavy construction soas to achieve the required homogeneous field) can be found only abovethe upper gradient coil and below the lower gradient coil. Consequently,the construction of this already bulky and heavy coil must be evenlarger; however, the outer coil must then also become larger. Moreover,in the case of actively shielded magnetic coils the shielding coils mustthen also become larger. The ultimate effect of the foregoing is thatthe costs of the apparatus are substantially increased again.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an apparatus of the kind setforth in which the distance between the outer coils and between thesupplementary coils is as small as possible. To achieve this, theapparatus in accordance with the invention is characterized in that

the energizing of the outer coil and of the supplementary coil is suchthat these coils generate magnetic fields of opposite direction,

the supplementary coil is also situated in the outer coil plane, and

the ratio D_(a)/D_(o) of the diameter D_(a) of the supplementary coil tothe diameter D_(o) of the outer coil is between 0.7 and 0.9.

Because the supplementary coil is now also situated in the outer coilplane, the distance between the supplementary coils is minimized whiletaking into account the required dimensions of the imaging volume. Acomputer simulation of this configuration has demonstrated that noconcessions need be made as regards the requirements in respect of fieldstrength and/or homogeneity of the main field when use is made of saidcombination of said energizing and said diameter ratio. It has also beenfound that an adequate degree of freedom exists as regards theconfiguration of the further coils, that is, in dependence on the exactshape, dimensions and energizing of the outer coil and the supplementarycoil.

An advantageous embodiment of the apparatus in accordance with theinvention is provided with three further round coils. It has been foundthat a suitable compromise can thus be achieved between production costs(condition: few and small further coils) and field strength andhomogeneity (condition: many and large further coils).

In a further advantageous embodiment in accordance with the inventionthe three further coils are situated on a conical surface, the apex ofthe conical surface being directed away from the imaging volume. Apartfrom the fact that this configuration very well satisfies therequirements as regards field strength and homogeneity, this embodimentnotably offers the advantage that there is created an inner space (thatis, a space around the vertical axis of the imaging volume) which,because of its conical shape, is very well compatible with a gradientcoil having a conical external appearance. This shape of a cavity in themagnetic pole, that is, in the cryo container of the magnetic coils,also has an additional advantage. In normal operating conditions thecooling medium present in a cryo container, that is, liquid helium, hasa pressure of approximately 1 bar. In given circumstances, however, thispressure may increase to as much as 3 bar. The helium container issurrounded by a vacuum space which, therefore, lies between the ambientatmosphere and the helium container. In the case of a cavity with squarecorners, as in the present state of the art, extreme mechanical stressescould occur at said pressures; when a more or less conical cavity isused, such stresses will occur to a much lesser extent because of thegradual shaping of the walls of the container.

A preferred embodiment of the MRI apparatus in accordance with theinvention is provided with a second field generating superconductingcoil system for producing the substantially homogeneous magnetic fieldin the imaging volume of the apparatus,

which second coil system includes:

a second round outer coil which is situated in a second outer coil planeand whose diameter is larger than that of the first outer coil,

a second round supplementary coil which is situated within the outercoil and in the outer coil plane,

the energizing of the second outer coil and of the second supplementarycoil being such that these coils generate magnetic fields of oppositedirection.

It is feasible to provide the coil system for generating the homogeneousfield with only one pole surface; in such apparatus a given concessionis made in respect of homogeneity and field strength, but itnevertheless remains possible to use the apparatus for given medicalpurposes. An apparatus of this kind is known, for example, from U.S.Pat. No. 5,917,395. When the apparatus is constructed so as to have twopole surfaces as is more usual, an attractive location of the imagingvolume relative to the pole surfaces can be chosen. This offers anadvantage in the following circumstances: a given size of the imagingvolume is defined in dependence on the amount of space desired for thepatient. This size defines the minimum distance between the polesurfaces. It should be possible to make optimum use of this distance forall imaging purposes, notably the imaging of parts of the body that aresituated at a low level in the imaging volume, for example the vertebralcolumn which is situated directly above the table top in the case ofpatients in the supine position. This table top, of course, should beconstructed so as to be as thin as possible as otherwise space thatcould be used for imaging is lost or the pole surfaces have to bearranged further apart again. The vertebral column is then situated atthe edge of the imaging volume, so that optimum homogeneity is notpossible over a great length. As a result of said steps (notably becausethe lower outer coil has a diameter which is larger than that of theupper coil), the imaging volume can be lowered relative to the polesurfaces, so that the vertebral column fits better in the imaging volumewithout it being necessary to enlarge the latter (which is expensive andleads to a high power consumption during operation).

A further embodiment of the apparatus in accordance with the inventionis provided with four further round coils, each of which is situated ina respective further coil plane, the outer coil plane being situatedbetween the imaging volume and each of the further coil planes. It hasbeen found that said number of further coils enables a suitablecompromise to be achieved between production costs, field strength andhomogeneity in the case of the desired lowering of the imaging volume.

In another embodiment yet of the invention the four further round coilsin the apparatus are situated on a conical surface, the apex of theconical surface being directed away from the imaging volume. Thus, thereis created a space for the main field within the coil container, theconical shape of said space being very compatible with a gradient coilhaving a conical external appearance.

Another embodiment of the apparatus in accordance with the invention isprovided with at least a first gradient coil system and a secondgradient coil system for producing a magnetic gradient field in theimaging volume of the apparatus,

each gradient coil system including a flat main gradient coil and ashielding coil, and

the first gradient coil system being situated in a space within thefirst field generating superconducting coil system and the secondgradient coil system being situated in a space within the second fieldgenerating superconducting coil system. The gradient coils are thussuitably arranged in the space created by the invention in the containerof the field coils for the homogeneous field.

At least one of the shielding coils in a further embodiment of theinvention extends across a substantially conical surface whose apex isdirected away from the imaging volume. It has been found that a gradientcoil is thus formed which produces a gradient field of the appearancerequired for MRI, notably in respect of linearity. The external shapethus obtained is particularly suitable for accommodating the gradientcoil in the cavity formed in the coil container in accordance with theinvention.

Another embodiment of the apparatus in accordance with the invention isprovided with a first and a second container for the first and thesecond field generating superconducting coil system, respectively, saidcontainers being arranged to contain a cryogenic medium andcommunicating with one another in order to exchange the cryogenicmedium, one of the containers being provided with a pressure connectionfor controlling the pressure in the containers as desired. In the caseof a system of superconducting coils it may occur that a part of thecoils does not to come into direct contact with the cooling medium(liquid helium), because a part thereof has evaporated and hence thecoils situated at the highest level have lost said contact.Consequently, said coils may come out of the superconducting state; thisis undesirable notably during operation. The entire system of coils, sothe contents of both coil containers, can now be cooled by means of onehelium system by making the lower container in this system serve also asa reservoir by storing more liquid helium therein than necessary for thelower coil holder alone. Should one of the coils in the upper coilcontainer tend to loose contact with the liquid medium, the pressure inthe lower container can be increased so that liquid is forced from thereservoir space to the upper container. The contact with the liquid isensured without it being necessary to use separate level control for therelevant container.

DESCRIPTION OF THE DRAWING FIGURES

The invention will be described in detail hereinafter with reference tothe Figures in which corresponding reference numerals denotecorresponding elements. Therein:

FIG. 1 is a general view of a known apparatus of the vertical field typefor the formation of MRI images;

FIG. 2 is a sectional view through the poles, the coil systems inaccordance with the invention being accommodated in the cryo containers;

FIG. 3 is a graphic representation of the field variation around theimaging volume of the apparatus in accordance with the invention;

FIG. 4a is a general view of the external appearance of a gradient coilsystem in accordance with the invention;

FIG. 4b shows the conductor pattern of the flat x main gradient coil ofthe gradient coil system in accordance with the invention;

FIG. 4c shows the conductor pattern of the x shielding coil of thegradient coil system in accordance with the invention;

FIG. 4d shows the conductor pattern of the flat z main gradient coil ofthe gradient coil system in accordance with the invention;

FIG. 4e shows the conductor pattern of the z shielding coil of thegradient coil system in accordance with the invention;

FIG. 4f is a side elevation of an x gradient coil system in accordancewith the invention, together with and the gradient field generated bythis system, and

FIG. 5 shows diagrammatically a cryo system that is suitable for use inthe apparatus in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a general view of a known the vertical field type apparatusfor the formation of MR images. The apparatus consists of a stand 2which supports the lower magnetic pole 4 and the upper magnetic pole 4.It is to be noted that in the present context a magnetic pole is to beunderstood to mean the assembly of associated field generating coils,without it being necessary (but possible) to provide an iron circuitwhich interconnects the two magnetic poles so as to conduct the magneticflux. A space for receiving a patient 8 to be examined exists betweenthe magnetic poles. The patient to be examined is arranged on a tabletop 14 which itself is supported by a support 16 which forms part of thestand 2 so that the patient 8 can be arranged in the correct positionand with the correct orientation between the magnetic poles 4 and 6.

The space for accommodating the patient to be examined in customary MRIapparatus is shaped as a tunnel having a cross-section of the order ofmagnitude of 60 cm; for many patients, notably children, thisconfiguration provokes feelings of anxiety and sensations ofclaustrophobia. The advantage of the constellation of magnetic poles asshown in FIG. 1 resides in the fact that the patient retains acomparatively broad view of the surroundings when arranged in such anapparatus, so that such feelings and sensations are alleviated or evendisappear.

At the side of the patient the magnetic poles are bounded by polesurfaces 10 and 12 which are physically formed as particular surfaceportions of upper and lower covers or outer vacuum containers 24 a, 24b, respectively, of helium or cryo containers 22 a, 22 b respectively,in which superconducting magnetic coil systems 20 a, 20 b, correspondingto the upper and lower poles, respectively, are accommodated. Thedistance between the pole surfaces is chosen to be such that saiduncomfortable sensations are counteracted for the patient, but not solarge that the production of the magnetic poles becomes much moreexpensive. It has been found in practice that a distance of from 50 to60 cm is a suitable value.

FIG. 2 is a sectional view through the magnetic poles 4 and 6,highlighting the coil systems 20 a and 20 b in accordance with theinvention. The coil systems are each arranged in the cryo containers, 20a, 20 b, each of which is further arranged in outer cover or vacuumcontainers 22 a, 22 b, respectively. The Figure shows the section of theround coil systems (that is, circular symmetrical systems around avertical line 15) with the plane of drawing; because of the circularsymmetry, this Figure shows only half of the coil systems, but the otherhalf may be assumed to be formed by mirror imaging relative to a planeextending through the line 15 and perpendicularly to the plane ofdrawing. Between the magnetic poles 4 and 6 there is situated a region18 in which the field generated by said magnetic poles is sufficientlyhomogeneous so as to form MRI images. This region is referred to as theimaging volume of the apparatus. Each of the magnetic poles 4 and 6includes field generating superconducting coil systems 20 a, 20 b,respectively, for producing a substantially homogeneous magnetic fieldin the imaging volume 18 of the apparatus (The coil system 20 a issituated in the upper magnetic pole 4 and the coil system 20 b issituated in the lower magnetic pole 6.) As is customary in the case ofsuperconducting coils, the coil systems are accommodated in the upperand lower helium containers, 20 a 20 b, respectively, which asmentioned, are themselves enclosed by the covers or outer vacuumcontainers 24 a, 24 b, respectively.

Each of the coil systems includes round outer coils 28 a, 28 b,respectively, and round supplementary coils 30 a, 30 b, which issupplemental coils are situated therewithin, respectively. Both outerand supplementary coils are situated in one flat plane, that is, upperand lower outer coil planes 32 a, 32 b, The ratio D_(a)/D_(o) of thediameter D_(a) of the supplementary coil to the diameter D_(o) of theouter coil generally lies between 0.7 and 0.9 and equals 0.8 in thepresent embodiment. In conformity with the idea of the invention, thedistance between the two outer coils 28 a and 28 b as well as thatbetween the two supplementary coils 30 a and 30 b can be minimized inthis configuration, meaning that the distance between these coils(ignoring the space for the vacuum space, the covers and the radiationshields that are not shown) is substantially equal to the distancebetween the pole surfaces 10 and 12. The comparatively high costs ofsaid coils can thus be limited to a minimum.

The upper coil system 20 a also includes three further round coils 34,36 and 38, each of which are situated in a respective further coil plane(not shown). Each of said further coil planes is situated further fromthe imaging volume 18 than the outer coil plane 32 a. In the embodimentshown the further coils 32 to 38 are situated on a conical surface 48whose section with the plane of drawing is shown, the apex of theconical surface 48 being directed away from the imaging volume, soupwards in the present case.

The lower coil system 20 b also includes four further round coils 40,42, 44 and 46, each of which is situated in a respective further coilplane (not shown). Each of said further coil planes is situated furtherfrom the imaging volume 18 than the outer coil plane 32 b. In theembodiment shown the further coils 40 to 46 are situated on a conicalsurface 50 whose section with the plane of drawing is shown, the apex ofthe conical surface 50 being directed away from the imaging volume, sodownwards in the present case.

It holds for both coil systems 20 a and 20 b that the energizing of theouter coils 28 a and 28 b is such that these coils produce a magneticfield of the same direction. The energizing of the supplementary coil 30a opposes that of the outer coil 28 a whereas the energizing of thesupplementary coil 30 b opposes that of the outer coil 28 b. The exactarrangement and energizing of the further coils 34 to 46 is aimed atfurther enhancement of the homogeneity of the magnetic field in theimaging volume 18; the arrangement of these coils has to be such that itenables the formation of a conical cavity in each of the magnetic poles.A gradient coil 52 a, 52 b can be arranged in each of said cavities,said gradient coils having a flat boundary at the side of the imagingvolume and a conical boundary at the side that is remote from theimaging volume.

FIG. 2 shows that the imaging volume 18 is not situated exactly halfwaybetween the pole surfaces 10 and 12 but has been lowered slightlyrelative to said center. As has already been described, this location ofthe imaging volume offers given advantages in respect of imaging of, forexample the vertebral column. The downward shift of the imaging volumeis achieved in that the outer field coil 28 b of the lower coil system20 b has a diameter which is larger than that of the outer field coil 28a of the upper coil system 20 a; a similar relationship exists betweenthe supplementary coils 30 a and 30 b. As a result of these steps, theimaging volume can be lowered relative to the pole surfaces, so that thevertebral column of a patient in the supine position in the imagingvolume fits better into the imaging volume, so that this volume need notbe enlarged.

Each of the coil systems also includes a round outer shielding coil 54a, 54 b and a round inner shielding coil 56 a and 56 b, respectively,which is situated within said outer shielding coils. These coils act inknown manner to shield the environment from the magnetic field generatedby the magnet coils.

FIG. 3 is a graphic representation of the field geometry around theimaging volume of the apparatus in accordance with the invention. Thehorizontal distance to the center of the imaging volume 18 is plotted onthe horizontal axis (the x axis) and the vertical distance to the centerof the imaging volume 18 is plotted on the vertical axis (the z axis).The Figure shows the coil systems 20 a and 20 b diagrammatically and toscale. The lines 58, 60, 62, 64, 66 and 68 in this Figure constitute thelines of equal field strength. The field strengths of 10 mT, 3 mT, 1 mT,0.3 mT, 0.1 mT and 0.05 mT, respectively, are associated with saidlines. Thus, it appears from this Figure that the field decays veryrapidly directly outside the imaging volume and that an increase of thedistance from 1.5 m (line 58) to 3.5 m (line 66) causes a field decreaseby even a factor of 100.

FIG. 4 shows the construction of a gradient coil system in the form of ashielded gradient coil that is suitable for use in the apparatus inaccordance with the invention. The gradient coil system consists of amain gradient coil for actually generating the gradient field and ashielding coil for compensating the gradient field as much as possibleoutside the imaging volume (notably at the area of the metal parts ofthe main magnet). The main gradient coil is wound in a flat planewhereas the shielding coil extends across a substantially conicalsurface whose apex is directed away from the imaging volume. Thisexternal appearance of a gradient coil can be used for the x gradientcoil as well as for the y gradient coil and the z gradient coil.However, the shape of the conductors of the x coil and the y coildeviates from that of the conductors of the z coil. The overall shape ofthe gradient coil system thus formed is shown in the side elevation ofFIG. 4a which is rotationally symmetrical around the line 70. The maingradient coil therein is denoted by the reference numeral 72 and theshielding coil is denoted by the reference numeral 74. A cylindricalreturn conductor (yet to be described) is denoted by the referencenumeral 76.

FIG. 4b shows the conductor pattern of the x and the y main gradientcoil 72; the x and the y coils have the same appearance, but are mountedso as to be 90° offset relative to one another. These conductorsgenerally have the shape of a number of concentric letters D which,moreover, are mirror imaged relative to one another. The concentric Dsare connected in series (not shown). The conductor patterns of theshielding coils 74 for the x gradient and the y gradient as shown inFIG. 4c have substantially the same appearance as those of the maingradient coil. However, these conductors are not arranged in a flatplane but on a conical surface whose apex height amounts toapproximately twice the height of a corresponding gradient coil systemhaving a flat shielding coil extending parallel to the main gradientcoil.

FIG. 4d shows the conductor pattern of the z main gradient coil 72.Generally speaking, these conductors have the shape of a number ofconcentric, non-equidistant circles. The concentric circles areconnected in series (not shown). The conductor pattern of the shieldingcoil 74 for the z gradient (FIG. 4e) has substantially the sameappearance as that of the main gradient coil 72. However, theseconductors are not arranged in a flat plane but on a conical surfacewhose apex height amounts to approximately twice the height of acorresponding gradient coil system having a flat shielding coilextending parallel to the main gradient coil.

The function of the return conductors of the main coil and those of theshielding coil can be combined in a manner that is known per se for thex gradient coil, the y gradient coil as well as the z gradient coil,with the result that return conductors can be dispensed with to asubstantial degree. The currents then flow from the main coil to theshielding coil via a cylindrical connection 76 on the side of thegradient coil system.

FIG. 4f is a side elevation of an x gradient coil; it also shows thedistribution of the gradient field of said coil. (The same applies tothe y gradient coil.) The straight arrows 78 therein represent the fieldstrength of the gradient field, that is, ∂B_(z)/∂x, and the curved lines80 represent the field lines of the gradient field. This Figure clearlyshows the gradient variation of the field, that is, the variation of thefield that is superposed on the magnetic main field and varies linearlyas a function of the location in the x direction.

FIG. 5 shows diagrammatically a cryo system that is suitable for use inthe apparatus in accordance with the invention. The system includes afirst, upper cryo container 82 and a second, lower cryo container 84which accommodate with field generating superconducting coil systems 86and 88. Each of the two cryo containers contains a cryogenic medium 92in the form of liquid helium. The two cryo containers 82 and 84communicate via a connection duct 90 for the exchange of the liquidhelium. The cryo container 84 is subdivided into two compartments 94 and96, the compartment 94 containing the coil system 88 whereas thecompartment 96 acts as a helium reservoir. The compartment 94 isprovided with a pressure connection 98 for controlling the pressure inthe containers as desired. When the coils in the upper coil containertend to loose contact with the liquid helium, the pressure on the lowercontainer can be increased via the pressure connection 98, so thatliquid helium is forced from the reservoir space 92 to the uppercontainer 82 via the compartment 96. The contact with the liquid heliumis thus ensured despite the absence of separate level control for theupper container.

What is claimed is:
 1. A vertical field type MRI apparatus for formingmagnetic resonance images, including: at least one field generatingsuperconducting coil system for producing a substantially homogeneousmagnetic field in an imaging volume of the apparatus, which coil systemincludes: a round outer coil arranged in an outer coil plane; and around supplementary coil arranged within the outer coil, wherein theenergizing of the outer coil and of the supplementary coil generatesmagnetic fields of opposite direction, wherein the supplementary coil islocated in the outer coil plane, and wherein the ratio D_(a)/D_(o) ofthe diameter D_(a) of the supplementary coil to the diameter D_(o) ofthe outer coil is between 0.7 and 0.9.
 2. An apparatus as claimed inclaim 1 and provided with three further round coils, each of which isarranged in a respective further coil plane, the outer coil planearranged between the imaging volume and each of the further coil planes.3. An apparatus as claimed in claim 2, wherein the three further roundcoils are arranged on a conical surface, the apex of the conical surfacebeing directed away from the imaging volume.
 4. An apparatus as claimedin claim 1 and provided with a second field generating superconductingcoil system for producing the substantially homogeneous magnetic fieldin the imaging volume of the apparatus, which second coil systemincludes: a second round outer coil arranged in a second outer coilplane and having a diameter that is larger than the diameter of thefirst outer coil, a second round supplementary coil arranged within thesecond outer coil and in the second outer coil plane, wherein theenergizing of the second outer coil and of the second supplementary coilgenerates magnetic fields of opposite direction.
 5. An apparatus asclaimed in claim 4 and provided with four further round coils, each ofwhich is arranged in a respective further coil plane, the outer coilplane located between the imaging volume and each of the further coilplanes.
 6. An apparatus as claimed in claim 5, wherein the four furtherround coils are arranged on a conical surface, the apex of the conicalsurface being directed away from the imaging volume.
 7. A vertical fieldtype MRI apparatus for forming magnetic resonance images, including: afirst field generating superconducting coil system for producing asubstantially homogeneous magnetic field in an imaging volume of theapparatus which coil systems includes: a round outer coil arranged in anouter coil plane; and a round supplementary coil arranged within theouter coil wherein the energizing of the outer coil and of thesupplementary coil generates magnetic fields of opposite direction,wherein the supplementary coil is located in the outer coil plane, andwherein the ratio D_(a)/D_(o) of the diameter D_(a) of the supplementarycoil to the diameter D_(o) of the outer coil is between 0.7 and 0.9; anda second field generating supperconducting coil system for producing thesubstantially homogeneous magnetic field in the imaging volume of theapparatus, which second coil system includes; a second round outer coilarranged in a second outer coil plane and having a diameter that islarger than the diameter of the first outer coil, a second roundsupplementary coil arranged within the second outer coil and in thesecond outer coil plane, wherein the energizing of the second outer coiland of the second supplementary coil generates magnetic fields ofopposite direction; wherein said vertical field type MRI apparatusfurther includes at least a first gradient coil system and a secondgradient coil system for producing a magnetic gradient field in theimaging volume of the apparatus, wherein each gradient coil systemincludes a flat main gradient coil and a shielding coil, and wherein thefirst gradient coil system is arranged in a space within said firstfield generating superconducting coil system and the second gradientcoil system is arranged in a space within the second field generatingsuperconducting coil system.
 8. An apparatus as claimed in claim 7,wherein at least one of the shielding coils extends across asubstantially conical surface whose apex is directed my from the imagingvolume.
 9. A vertical field type MRI apparatus for forming magneticresonance images, including; a first field generating supperconductingcoil system for producing a substantially homogeneous magnetic field inan imagine volume of the apparatus, which coil sytem includes: a roundouter coil arranged in an outer coil plane; and a round supplementarycoil arranged within the outer coil, wherein the energizing of the outercoil and of the supplementary coil generates magnetic fields of oppositedirection, wherein the supplementary coil is located in the outer coilplane, and wherein the ratio D_(o)/D_(a) of the diameter D_(o) of thesupplementary coil to the diameter D_(o) of the outer coil is between0.7 and 0.9; and a second field generating supperconducting coil systemfor producing the substantially homogeneous magnetic field in theimaging volume of the apparatus, which second coil system includes; asecond round outer coil arranged in a second outer coil plane and havinga diameter that is larger than the diameter of the first outer coil, asecond round supplementary coil arranged within the second outer coiland in the second outer coil plane wherein the energizing of the secondouter coil and of the second supplementary coil generates magneticfields of opposite direction; wherein said vertical field type MRJapparatus further includes a first and a second container for the firstand the second field generating superconducting coil system,respectively, said containers being arranged to contain a cryogenicmedium and communicating with one another in order to exchange thecryogenic medium, one of the containers being provided with a pressureconnection for controlling the pressure in the containers as desired.